Imaging lens

ABSTRACT

An imaging lens includes an aperture stop, a positive first lens with a biconvex shape, a negative second lens; a negative third lens, a positive fourth lens, and a negative fifth lens arranged in this order from an object side. When the whole lens system has a focal length f, focal lengths and Abbe&#39;s numbers of the first and the second lenses are f1, νd1, f2, and νd2, focal lengths of the fourth and fifth lenses are f4 and f5, a composite focal length of the first lens L 1  and the second lens L 2  is f12, and a distance from a surface of the first lens L 1  on the object side to a surface of the fifth lens L 5  on the image side is Σd, the imaging lens satisfies the following conditional expressions:
 
0.7&lt;f12/f&lt;1.4
 
0.2&lt;|f1/f2|&lt;0.6
 
15&lt;νd1−νd2
 
0.4&lt;f4/f&lt;1.0
 
Σd/f&lt;1.2
 
|f5/f|&lt;1.0

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation reissue application of theapplication Ser. No. 14/642,942, which is an application for reissue ofU.S. Pat. No. 8,411,376.

This is a continuation application of the prior PCT applicationPCT/JP2009/006799, filed on Dec. 11, 2009, pending, which claimspriority from a Japanese patent application No. 2008-329285, filed onDec. 25, 2008, the entire content of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image onan imaging element such as a CCD sensor and a CMOS sensor. Inparticular, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a cellular phone, adigital still camera, a portable information terminal, a securitycamera, an onboard camera, and a network camera.

An imaging lens to be mounted in a small camera has been required tohave a high resolution lens configuration suitable for a recentlydeveloped imaging element with a high resolution, as well as to use afewer number of lenses. Conventionally, a three-lens imaging lens hasbeen frequently used as such an imaging lens. However, as an imagingelement has higher resolution, it is more difficult to obtain sufficientperformances only with three lenses. In these years, another lensconfiguration, a four-lens configuration or a five-lens configuration,has been applied.

Among the configurations, since a configuration with five lenses has ahigher design flexibility, it may be expected to apply such lensconfiguration in a next-generation imaging lens. An imaging lensdisclosed in Patent Reference has been known as an imaging lens havingsuch a five-lens configuration.

The imaging lens disclosed in Patent Reference includes a positive firstlens having a convex surface on the object side; a second lens having anegative meniscus shape that directs a concave surface on the imageside; a third lens having a positive meniscus shape that directs aconvex surface on the image side; a negative fourth lens in which bothsurfaces have an aspheric shape and a surface thereof on the image sidenear an optical axis is concave; and a positive or negative fifth lens,in which both surfaces are aspheric shape, in this order from the objectside.

In this configuration, when a lower limit of Abbe's number of the firstlens and upper limits of Abbe's numbers of the second and the fourthlens are respectively assigned, an axial chromatic aberration andchromatic aberration of magnification are corrected, so as to compatiblewith a high performance imaging lens.

Patent Reference Japanese Patent Application Publication No. 2007-264180

According to the imaging lens disclosed in Patent Reference, it ispossible to obtain relatively satisfactory aberrations. Since the totallength of the lens system is long, however, it is difficult to attainboth miniaturization of an imaging lens and satisfactory aberrationcorrection.

In view of the problems of the conventional techniques described above,an object of the present invention is to provide an imaging lens with asmall size capable of properly correcting aberration.

SUMMARY OF THE INVENTION

In order to attain the object described above, according to the presentinvention, an imaging lens includes a first lens having positiverefractive power; a second lens having negative refractive power; athird lens having negative refractive power; a fourth lens havingpositive refractive power; and a fifth lens having negative refractivepower in this order from the object side to the image side. The firstlens is shaped to form a biconvex lens and the second lens is shaped toform a lens that directs a concave surface on the object side.

According to the invention, the first lens is shaped to form a biconvexlens. Therefore, it is possible to set the refractive power of the firstlens relatively strong, so that it is possible to suitably attainminiaturization of an imaging lens. On the other hand, with this firstlens having positive refractive power, there remains a concern ofgeneration of field curvature. For this reason, according to theinvention, disposing on the image side of the first lens the second lenshaving negative refractive power so as to direct the concave surface onthe object side, it is possible to reduce worsening of the fieldcurvature generated at the first lens. Therefore, according to theimaging lens of this invention, despite the small size, it is possibleto satisfactorily correct the aberrations.

Here, for a shape of the third lens, for example, it may be possible tochoose a shape of a meniscus lens that directs a concave surface on theobject side. In addition, as a shape of the fourth lens, for example, itmay be possible to choose a shape of a biconvex lens.

According to the imaging lens with the above-described configuration,when the whole lens system has a focal length f and a composite focallength of the first lens and the second lens is f12, it is preferred tosatisfy the following conditional expression (1):0.7<f12/f<1.4   (1)

When the above conditional expression (1) is satisfied, it is possibleto keep the total length of the imaging lens short and also the fieldcurvature and coma aberration stable. When the value exceeds the upperlimit “1.4”, the focal length of the first lens increases, so that it isdifficult to attain a small-sized imaging lens. On the other hand, if itis below the lower limit “0.7”, the refractive power of the first lensis too strong, so that it is difficult to secure the back focal length.In order to secure a certain back focal length, it is necessary toincrease the refractive power of the third lens. When the value is belowthe lower limit “0.7”, even if it is possible to attain a small-sizedimaging lens, it is difficult to correct the field curvature and correctcoma aberration, so that it is difficult to attain both a small-sizedimaging lens and satisfactory aberration correction.

Furthermore, according to the imaging lens with the aforementionedconfiguration, when the first lens has a focal length f1 and the secondlens has a focal length f2, it is preferred to satisfy the followingconditional expression (2):0.2<|f1/f2|<0.6   (2)

When the above conditional expression (2) is satisfied, it is possibleto keep the axial chromatic aberration and spherical aberration stable.When the value exceeds the upper limit “0.6”, since the refractive powerof the second lens increases, the axial chromatic aberration is in theplus direction in relative to that of a reference wavelength and isexcessively corrected. In addition, the spherical aberration is in theplus direction at a ring zone section and is excessively corrected.

As a result, it is difficult to keep the axial chromatic aberration andspherical aberration stable. On the other hand, when the value is belowthe lower limit “0.2”, since the refractive power of the second lensdecreases, the axial chromatic aberration is in the minus direction inrelative to that of the reference wavelength and is insufficientlycorrected. In addition, even the spherical aberration is in the minusdirection at the ring zone section and is similarly insufficientlycorrected. Therefore, even in this case, it is difficult to keep theaxial chromatic aberration and spherical aberration stable, and it isdifficult to obtain satisfactory imaging performance.

Moreover, in case of the imaging lens with the aforementionedconfiguration, when Abbe's number of the first lens is νd1 and Abbe'snumber of the second lens is νd2, it is more preferred to satisfy thefollowing conditional expression (3):15<νd1−νd2   (3)

When the above conditional expression (3) is satisfied, it is possibleto keep the axial chromatic aberration and off-axis chromatic aberrationstable while satisfactorily correcting those chromatic aberrations. Ifthe conditional expression (3) is not satisfied, the axial chromaticaberrations at short wavelengths increase in the minus direction inrelative to that of the reference wavelength, and the aberration isinsufficiently corrected.

When the Abbe's number of the third lens is set to a small value inorder to improve such insufficient correction of chromatic aberration,the axial chromatic aberration is satisfactorily corrected, but theoff-axis chromatic aberration of magnification is excessively correctedand worsened.

Further, in case of the imaging lens with the aforementionedconfiguration, when the whole lens system has a focal length f and thefourth lens has a focal length f4, it is more preferred to satisfy thefollowing conditional expression (4):0.4<f4/f<1.0   (4)

In case of an imaging element such as a CCD sensor and a CMOS sensor,there is a limit in an acceptance angle of an incoming light beam due toits structure. Generally speaking, this limit in the acceptance angle ofan incoming light beam is provided as certain range around principallight beam (e.g. ±25° of the principal light beam).

When an angle of emergence of the off-axis principal light beam isoutside the limitation range, since a sensor does not take a light beamoutside the range therein, a resultant image taken through the imaginglens has a periphery that is dark in comparison with a center part. Inother words, a shading phenomenon occurs.

When the aforementioned conditional expression (4) is satisfied, it ispossible to keep the maximum angle of emergence of the off-axisprincipal light beam small while keeping each aberration stable. Whenthe value exceeds the upper limit “1.0”, since the refractive power ofthe fourth lens decreases, while it is easy to correct the comaaberration and the chromatic aberration of magnification, the maximumangle of emergence of the off-axis principal light beam becomes largeand a shading phenomenon more easily occurs. On the other hand, when thevalue is below the lower limit “0.4”, since the refractive power of thefourth lens increases, although it is possible to reduce the maximumangle of emergence of the off-axis principal light beam, it is difficultto correct the field curvature and the distortion.

Moreover, in the imaging lens with the aforementioned configuration,when the whole lens system has a focal length f and a distance on theoptical axis from a surface of the first lens on the object side to asurface of the fifth lens on the image side is Σd, it is preferred tosatisfy the following conditional expression (5) also in view ofminiaturization of an imaging lens:Σd/f<1.2   (5)

In addition, in the imaging lens with the aforementioned configuration,when the whole lens system has a focal length f and the fifth lens has afocal length f5, it is preferred to satisfy the following conditionalexpression (6):|f5/f|<1.0   (6)

As well known, as an effective means to attain miniaturization of animaging lens, it may be possible to reduce a focal length of a lens.Actually, this approach has been employed in designing many imaginglenses. However, when a focal length decreases while keeping an idealimage height constant, the angle of emergence of the off-axis light beamincreases, and it is more difficult to balance among aberrationsincluding a spherical aberration, a chromatic aberration, distortion,and a field curvature. Therefore, it is necessary to attainminiaturization of an imaging lens while keeping the focal length long.

When the imaging lens has a configuration that satisfies the conditionalexpression (6), a position of a principal point of the optical systemmoves towards the object side, so that it is possible to attainminiaturization of an imaging lens while keeping the focal length long.Here, it is effective to satisfy the conditional expression (5) also asa means to supplement insufficient correction of axial chromaticaberration.

According to the imaging lens of the invention, it is possible to bothreduce the size of the imaging lens and correct the aberration properly,thereby making it possible to provide the imaging lens with the smallsize capable of correcting aberrations properly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 1;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens in Numerical Data Example 1;

FIG. 3 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens in Numerical DataExample 1;

FIG. 4 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 2;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens in Numerical Data Example 2;

FIG. 6 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens in Numerical DataExample 2;

FIG. 7 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 3;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens in Numerical Data Example 3;

FIG. 9 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens in Numerical DataExample 3;

FIG. 10 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 4;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens in Numerical Data Example 4;

FIG. 12 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens in Numerical DataExample 4;

FIG. 13 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 5;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens in Numerical Data Example 5; and

FIG. 15 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens in Numerical DataExample 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

Hereunder, referring to the accompanying drawings, a first embodiment ofthe present invention will be fully described.

FIGS. 1, 4, and 7, and 10 are schematic sectional views showing imagelenses in Numerical Data Examples 1 to 4 according to the embodiment,respectively. Since a basic lens configuration is the same among theNumerical Data Examples 1 to 4, the lens configuration of theembodiments will be described with reference to the lens sectional viewof Numerical Data Example 1.

As shown in FIG. 1, the imaging lens of the embodiment includes anaperture stop ST; a first lens L1 having positive refractive power; asecond lens L2 having negative refractive power; a third lens L3 havingnegative refractive power; a fourth lens L4 having positive refractivepower; and a fifth lens L5 having negative refractive power, which arearranged in this order from an object side to an image side of theimaging lens. A cover glass 10 is provided between the fifth lens L5 andthe image plane of an imaging element. It is noted that the cover glass10 may be optionally omitted.

In the imaging lens with the above-described configuration, the firstlens L1 is a biconvex lens, and the second lens L2 is a meniscus lensthat directs a concave surface on the object side. These first lens L1and the second lens L2 satisfy the following conditional expressions (1)to (3):0.7<f12/f<1.4   (1)0.2<|f1/f2|<0.6   (2)15<νd1−νd2   (3)

In the above conditional expressions,

f: Focal length of the whole lens system

f1: Focal length of the first lens L1

f2: Focal length of the second lens L2

f12: Composite focal length of the first lens L1 and the second lens L2

νd1: Abbe's number of the first lens L1

νd2: Abbe's number of the second lens L2

When the conditional expressions (1) to (3) are satisfied, it ispossible to obtain the following effects respectively. When theconditional expression (1) is satisfied, it is possible to keep thefield curvature and coma aberration stable while keeping the wholelength of the imaging lens short. In addition, when the conditionalexpression (2) is satisfied, it is possible to keep the axial chromaticaberration and spherical aberration stable. Furthermore, when theconditional expression (3) is satisfied, it is possible to keep theaxial chromatic aberration and off-axis chromatic aberration stablewhile properly correcting those chromatic aberrations.

In such configuration, according to this embodiment, the third lens L3is shaped to form a meniscus lens that directs a concave surface on theobject side and the fourth lens L4 is shaped to form a biconvex lens.

The fifth lens L5 is shaped to form a biconcave lens. In this fifth lensL5, a surface thereof on the image side is shaped to form an asphericshape, which is concaved on the image side near the optical axis and isconvex on the image side at the periphery, i.e. aspheric shape having aninflection point. Because of this, an incident angle of a light beamemitted from the fifth lens L5 to an image plane is restrained.

In the embodiment, the lens surfaces of all lenses are formed to be anaspheric surface as necessary.

When the aspheric surface applied to the lens surfaces have an axis Z inthe optical axis direction, a height H in a direction perpendicular tothe optical axis, a conical coefficient k, and the aspheric coefficientsA₄, A₆, A₈, and A₁₀, the aspheric surfaces of the lens surfaces may beexpressed as follows. Here, even in case of an imaging lens according toa second embodiment, which will be described later, the lens surfaces ofall lenses are formed to be an aspheric surface as necessary, andaspheric surface shapes applied in theses lens surfaces are expressed bythe following formula similarly to this embodiment:

$\begin{matrix}{Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$The imaging lens according to this embodiment satisfies the followingconditional expressions (4) to (6) in addition to the aforementionedconditional expressions (1) to (3):0.4<f4/f<1.0   (4)Σd/f<1.2   (5)|f5/f|<1.0   (6)

In the above conditional expressions,

f: Focal length of the whole lens system

f4: Focal length of the fourth lens L4

f5: Focal length of the fifth lens L5

Σd: Distance on the optical axis from a surface of the first lens L1 onthe object side to a surface of the fifth lens L5 on the image side.

When the conditional expressions (4) to (6) are satisfied, it ispossible to obtain the following effects respectively. When theconditional expression (4) is satisfied, it is possible to keep themaximum angle of emergence of the off-axis principal light beam small,while keeping each aberration stable. In addition, when the conditionalexpression (5) is satisfied, it is possible to attain miniaturization ofthe imaging lens. Furthermore, when the conditional expression (6) issatisfied, it is possible to attain miniaturization of the imaging lenswhile keeping the focal length long.

Here, it is not necessary to satisfy all of the above conditionalexpressions (1) to (6). When any single one of the conditionalexpressions (1) to (6) is individually satisfied, it is possible toobtain an effect corresponding to the respective conditional expression.

Next, Numerical Data Examples of the embodiment will be described. Ineach of Numerical Data Examples, f represents a focal length of a wholelens system, Fno represents an F number, and ω represents a half angleof view, respectively. In addition, i represents a surface numbercounted from the object side, R represents a curvature radius, drepresents a distance between lens surfaces (an on-axis surface spacing)along the optical axis, Nd represents a refractive index for a d line,and νd represents Abbe's number at the d line. Here, the asphericsurfaces are indicated with surface numbers affixed with * (asterisk).

Numerical Data Example 1

Basic lens data are shown below. f = 3.903 mm, Fno = 2.805, ω = 31.59°Unit: mm Surface Data Surface Number i R d Nd νd (Object) ∞ ∞ 1  ∞ 0(Stop) 2* 1.571 0.5500 1.52470 56.2 (=νd1) 3* −7.132 0.1500 4  −3.5210.3000 1.61420 26.0 (=νd2) 5* −19.595 0.2800 6* −1.823 0.2800 1.5850029.0 7* −5.912 0.3600 8* 3.357 0.8500 1.52470 56.2 9* −1.613 0.3000 10* −2.617 0.3300 1.52470 56.2 11*  3.067 0.3000 12  ∞ 0.1500 1.51633 64.1213  ∞ 0.8823 (Image plane) ∞ f1 = 2.508 f2 = −7.038 f12 = 3.573 f4 =2.206 f5 = −2.639 Σd = 3.400 Aspheric Surface Data Second Surface k =−1.544427E−01, A₄ = 1.976046E−02, A₆ = −1.793809E−02 Third Surface k =1.063940, A₄ = 4.713006E−03, A₆ = −2.945120E−02 Fifth Surface k =1.415349E+02, A₄ = −2.371673E−02, A₆ = 1.311554E−02 Sixth Surface k =−2.723790, A₄ = −1.147380E−02, A₆ = −4.130846E−02, A₈ = −3.948624E−03,A₁₀ = 5.021037E−02 Seventh Surface k = −9.933691, A₄ = 3.758781E−03, A₆= 1.719515E−02, A₈ = 1.736953E−02, A₁₀ = 1.092378E−02 Eighth Surface k =−2.241293E+01, A₄ = −2.578027E−02, A₆ = −7.694008E−03, A₈ =−1.375408E−03, A₁₀ = 6.496087E−04 Ninth Surface k = 6.248352E−02, A₄ =1.165596E−01, A₆ = −3.911326E−02, A₈ = 1.261679E−02, A₁₀ = 1.134638E−04Tenth Surface k = 1.999301, A₄ = 3.503592E−02, A₆ = −3.907364E−02, A₈ =1.551177E−02, A₁₀ = −3.231912E−03 Eleventh Surface k = 2.223974E−01, A₄= −9.602329E−02, A₆ = 7.338596E−03, A₈ = −1.181135E−03, A₁₀ =−3.315528E−04 Values of the conditional expressions (1) to (6) are asfollows: f12/f = 0.915 |f1/f2| = 0.356 νd1 − νd2 = 30.2 f4/f = 0.565Σd/f = 0.871 |f5/f| = 0.676

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theconditional expressions (1) to (6). FIG. 2 shows the lateral aberrationthat corresponds to the half angle of view ω in the imaging lens ofNumerical Data Example 1 by dividing into a tangential direction andsagittal direction (which is also the same in FIGS. 5, 8, and 11).Furthermore, FIG. 3 shows a spherical aberration SA (mm), an astigmatismAS (mm), and a distortion DIST (%), respectively. In the aberrationdiagrams, the Offence against the Sine Condition (OSC) is also indicatedfor the spherical aberration diagram in addition to the aberrations atthe respective wavelengths of 587.56 nm, 435.84 nm, 656.27 nm, 486.13nm, and 546.07 nm. Further, in the astigmatism diagram, the aberrationon the sagittal image surface S and the aberration on the tangentialimage surface T are respectively indicated (which are the same in FIGS.6, 9, and 12).

As shown in FIGS. 2 and 3, in the imaging lens of Numerical Data Example1, the respective aberrations are satisfactorily corrected. Especially,as shown in the astigmatism diagram, the astigmatic difference is verysmall, the image surface is satisfactorily corrected, and the distortionis also small.

Numerical Data Example 2

Basic lens data are shown below. f = 3.899 mm, Fno = 2.800, ω = 32.67°Unit: mm Surface data Surface Number i R d Nd νd (Object) ∞ ∞ 1  ∞ 0(Stop) 2* 1.604 0.5500 1.52470 56.2 (=νd1) 3* −8.437 0.1500 4  −3.4560.3000 1.61420 26.0 ( =νd2) 5* −18.051 0.2800 6* −1.948 0.2800 1.5850029.0 7* −4.589 0.3000 8* 3.916 0.8000 1.52470 56.2 9* −1.597 0.2500 10* −2.618 0.3300 1.52470 56.2 11*  3.256 0.3000 12  ∞ 0.1500 1.51633 64.1213  ∞ 1.0397 (Image plane) ∞ f1 = 2.618 f2 = −7.014 f12 = 3.817 f4 =2.276 f5 = −2.713 Σd = 3.240 Aspheric Surface Data Second Surface k =−1.439472E−01, A₄ = 2.032875E−02, A₆ = −1.399217E−02 Third Surface k =1.913498, A₄ = 4.854973E−03, A₆ = −1.585207E−02 Fifth Surface k =2.521344E+02, A₄ = −2.591410E−02, A₆ = 3.798179E−03 Sixth Surface k =−3.165513, A₄ = −7.669636E−03, A₆ = −4.014852E−02, A₈ = 5.980691E−03,A₁₀ = 3.235352E−02 Seventh Surface k = −3.803800, A₄ = 1.097028E−03, A₆= 1.770991E−02, A₈ = 1.736123E−02, A₁₀ = 1.453023E−02 Eighth Surface k =−5.618736E+01, A₄ = −3.829839E−02, A₆ = −1.530875E−02, A₈ =−3.059261E−03, A₁₀ = 4.242948E−04 Ninth Surface k = 8.069668E−02, A₄ =1.050962E−01, A₆ = −4.000834E−02, A₈ = 1.262215E−02, A₁₀ = −9.911318E−05Tenth Surface k = 2.019736, A₄ = 4.417573E−02, A₆ = −3.888932E−02, A₈ =1.333875E−02, A₁₀ = −4.117009E−03 Eleventh Surface k = 8.429077E−01, A₄= −9.283294E−02, A₆ = 9.981823E−03, A₈ = −1.869262E−03, A₁₀ =−4.558872E−04 Values of each conditional expression are as follows:f12/f = 0.979 |f1/f2| = 0.373 νd1 − νd2 = 30.2 f4/f = 0.584 Σd/f = 0.831|f5/f| = 0.696

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theconditional expressions (1) to (6). FIG. 5 shows the lateral aberrationthat corresponds to the half angle of view ω in the imaging lens ofNumerical Data Example 2, and FIG. 6 shows the spherical aberration SA(mm), the astigmatism AS (mm), and the distortion DIST (%),respectively. As shown in FIGS. 5 and 6, in the imaging lens ofNumerical Data Example 2, the image surface is satisfactorily corrected,and the respective aberrations are satisfactorily corrected similarly toNumerical Data Example 1.

Numerical Data Example 3

Basic lens data are shown below. f = 3.907 mm, Fno = 2.805, ω = 32.64°Unit: mm Surface Data Surface Number i R d Nd νd (Object) ∞ ∞ 1  ∞ 0(Stop) 2* 1.575 0.5500 1.52470 56.2 ( = vd1) 3* −7.276 0.1500 4  −3.5020.3000 1.61420 26.0 ( = vd2) 5* −19.883 0.2800 6* −1.831 0.2800 1.5850029.0 7* −5.761 0.3600 8* 3.400 0.8500 1.52470 56.2 9* −1.611 0.3000 10* −2.619 0.3300 1.52470 56.2 11*  3.153 0.3000 12  ∞ 0.1500 1.51633 64.1213  ∞ 0.9013 (Image plane) ∞ f1 = 2.521 f2 = −6.969 f12 = 3.615 f4 =2.212 f5 = −2.674 Σd = 3.400 Aspheric Surface Data Second Surface k =−1.709914E−01, A₄ = 1.904787E−02, A₆ = −1.792465E−02 Third Surface k =7.046181, A₄ = 2.542724E−03, A₆ = −2.853682E−02 Fifth Surface k =1.341757E+02,, A₄ = −2.342623E−02 A₆ = 1.151604E−02 Sixth Surface k =−2.664785, A₄ = −1.221495E−02, A₆ = −4.161820E−02, A₈ = −3.480280E−03,A₁₀ = 4.712500E−02 Seventh Surface k = −1.007092E+01, A₄ = 3.857254E−03,A₆ = 1.729519E−02, A₈ = 1.721151E−02, A₁₀ = 1.079714E−02 Eighth Surfacek = −2.440426E+01, A₄ = −2.565446E−02, A₆ = −8.232621E−03, A₈ =−1.561612E−03, A₁₀ = 6.144595E−03 Ninth Surface k = 6.497601E−02, A₄ =1.149186E−01, A₆ = −3.897702E−02, A₈ = 1.270717E−02, A₁₀ = 1.210040E−04Tenth Surface k = 1.994817, A₄ = 3.657337E−02, A₆ = −3.934563E−02, A₈ =1.507533E−02, A₁₀ = −3.504426E−03 Eleventh Surface k = 3.526177E−02, A₄= −9.652400E−02, A₆ = 7.275239E−03, A₈ = −1.425736E−03, A₁₀ =−3.842309E−04 Values of the conditional expressions (1) to (6) are asfollows: f12/f = 0.925 |f1/f2| = 0.362 νd1 − νd2 = 30.2 f4/f = 0.566Σd/f = 0.870 |f5/f| = 0.684

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theconditional expressions (1) to (6). FIG. 8 shows the lateral aberrationthat corresponds to the half angle of view ω in the imaging lens ofNumerical Data Example 3, and FIG. 9 shows the spherical aberration SA(mm), the astigmatism AS (mm), and the distortion DIST (%),respectively. As shown in FIGS. 8 and 9, in the imaging lens ofNumerical Data Example 3, the image surface is satisfactorily corrected,and the respective aberrations are satisfactorily corrected similarly toNumerical Data Example 1.

Numerical Data Example 4

Basic lens data are shown below. f = 3.848 mm, Fno = 2.805, ω = 30.32°Unit: mm Surface Data Surface Number i R d Nd νd (Object) ∞ ∞ 1  ∞ 0(Stop) 2* 1.566 0.5500 1.52470 56.2 (=νd1) 3* −7.023 0.1500 4  −3.5360.3000 1.61420 26.0 (=νd2) 5* −19.676 0.2800 6* −1.837 0.2800 1.5247056.2 7* −6.033 0.3600 8* 3.302 0.8500 1.52470 56.2 9* −1.614 0.3000 10* −2.639 0.3300 1.58500 29.0 11*  2.922 0.3000 12  ∞ 0.5000 1.51633 64.1213  ∞ 0.5310 (Image plane) ∞ f1 = 2.495 f2 = −7.068 f12 = 3.539 f4 =2.197 f5 = −2.320 Σd = 3.400 Aspheric Surface Data Second Surface k =−1.514551E−01, A₄ = 2.041462E−02, A₆ = −2.332746E−02 Third Surface k =4.107515E−02, A₄ = 5.196393E−03, A₆ = −3.559135E−02 Fifth Surface k =8.917948E+01, A₄ = −2.298304E−02, A₆ = 1.782019E−02 Sixth Surface k =−2.682345, A₄ = −1.244876E−02, A₆ = −4.412886E−02, A₈ = −8.375465E−03,A₁₀ = 5.365573E−02 Seventh Surface k = −1.211874E+01, A₄ = 4.641584E−03,A₆ = 1.781682E−02, A₈ = 1.808663E−02, A₁₀ = 1.095189E−02 Eighth Surfacek = −1.915006E+01, A₄ = −2.420006E−02, A₆ = −6.347979E−03, A₈ =−8.744578E−04, A₁₀ = 8.120666E−04 Ninth Surface k = 6.472730E−02, A₄ =1.202224E−01, A₆ = −3.910233E−02, A₈ = 1.241952E−02, A₁₀ = 4.587975E−05Tenth Surface k = 1.948054, A₄ = 3.123665E−02, A₆ = −3.881734E−02, A₈ =1.645138E−02, A₁₀ = −2.866062E−03 Eleventh Surface k = 1.218782, A₄ =−9.899569E−02, A₆ = 7.341671E−03, A₈ = −9.793363E−04, A₁₀ =−3.286655E−04 Values of the conditional expressions (1) to (6) are asfollows: f12/f = 0.920 |f1/f2| = 0.353 νd1 − νd2 = 30.2 f4/f = 0.571Σd/f = 0.884 |f5/f| = 0.603

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theconditional expressions (1) to (6). FIG. 11 shows the lateral aberrationthat corresponds to the half angle of view ω in the imaging lens ofNumerical Data Example 4, and FIG. 12 shows the spherical aberration SA(mm), the astigmatism AS (mm), and the distortion DIST (%),respectively. As shown in FIGS. 11 and 12, in the imaging lens ofNumerical Data Example 4, the image surface is satisfactorily corrected,and the respective aberrations are satisfactorily corrected similarly toNumerical Data Example 1.

Second Embodiment

Hereunder, referring to the accompanying drawings, a second embodimentof the invention will be described. Similarly to the imaging lens of thefirst embodiment, the imaging lens of this embodiment includes anaperture stop ST; a first lens L1 having positive refractive power; asecond lens L2 having negative refractive power; a third lens L3 havingnegative refractive power; a fourth lens L4 having positive refractivepower; and a fifth lens L5 having negative refractive power, which arearranged in this order from the object side towards the image side of animaging lens. A cover glass 10 is provided between the fifth lens L5 andthe image plane.

According to the imaging lens of this embodiment, however, the secondlens L2 is a biconcave lens, the first lens L1 and the second lens L2are combined as shown in FIG. 13. With the lens configuration like this,it is possible to more suitably correct chromatic aberration.

More specifically, in the imaging lens of this embodiment, the firstlens L1 is biconvex lens, the second lens L2 is a biconcave lens, andthose lenses are combined. The third lens L3 is a meniscus lens thatdirects a concave surface on the object side, and the fourth lens L4 isa biconvex lens. The fifth lens L5 is a biconcave lens, and a surfacethereof on the image side is formed to be an aspheric shape having aninflection point.

Even in this embodiment, the imaging lens is configured to satisfy thefollowing conditional expressions (1) to (6) similarly to the firstembodiment.0.7<f12/f<1.4   (1)0.2<|f1/f2|<0.6   (2)15<νd1−νd2   (3)0.4<f4/f<1.0   (4)Σd/f<1.2   (5)|f5/f|<1.0   (6)

In the above conditional expressions,

f: Focal length of the whole lens system

f1: Focal length of the first lens L1

f2: Focal length of the second lens L2

f12: Composite focal length of the first lens L1 and the second lens L2

νd1: Abbe's number of the first lens L1

νd2: Abbe's number of the second lens L2

f4: Focal length of the fourth lens L4

f5: Focal length of the fifth lens L5

Σd: Distance on the optical axis from a surface of the first lens L1 onthe object side to a surface of the fifth lens L5 on the image side.

Next, Numerical Data Examples of the imaging lens according to thisembodiment are shown. In this Numerical Data Example, f is a focallength of the whole lens system, Fno represents an F number, and ωrepresents a half angle of view, respectively. Moreover, i represents asurface number counted from the object side, R represents a curvatureradius, d is a distance between lens surfaces, Nd on the optical axis isrefractive index for a d line, and νd is Abbe's number at a d line,respectively. Here, the aspheric surfaces are indicated with surfacenumbers affixed with * (asterisk).

Numerical Data Example 5

Basic lens data are shown below. f = 3.871 mm, Fno = 2.800, ω = 30.17°Unit: mm Surface Data Surface Number i R d Nd νd (Object) ∞ ∞ 1  ∞ 0(Stop) 2* 2.393 0.5000 1.67790 55.5 (=νd1) 3  −3.334 0.3000 1.66446 36.0(=νd2) 4  41.657 0.4272 5* −1.904 0.2640 1.58500 29.0 6* −4.245 0.66957* 4.525 0.8798 1.52470 56.2 8* −1.707 0.4993 9* −2.515 0.3234 1.5850029.0 10*  3.495 0.1000 11  ∞ 0.5000 1.51633 64.12 12  ∞ 0.5945 (Imageplane) ∞ f1 = 2.130 f2 = −4.633 f12 = 3.660 f4 = 2.483 f5 = −2.451 Σd =3.863 Aspheric Surface Data Second Surface k = 2.191111, A₄ =−9.435404E−03, A₆ = −3.285189E−02, A₈ = 3.672070E−02, A₁₀ =−3.270308E−02 Fifth Surface k = −1.984333, A₄ = −1.805977E−02, A₆ =−2.675334E−02, A₈ = 7.933052E−04, A₁₀ = 1.631158E−02 Sixth Surface k =−5.661022E−01, A₄ = −1.709355E−02, A₆ = −1.717627E−03, A₈ =−5.550278E−03, A₁₀ = 4.049249E−03 Seventh Surface k = −4.220498, A₄ =−8.6408444E−03, A₆ = −8.170485E−04, A₈ = −6.511240E−05, A₁₀ =2.033590E−05 Eighth Surface k = 3.300032E−02, A₄ = 1.125967E−01, A₆ =−3.672376E−02, A₈ = 1.170047E−02, A₁₀ = 3.409990E−04 Ninth Surface k =1.781895, A₄ = 6.458725E−03, A₆ = −5.275346E−02, A₈ = 1.858108E−02, A₁₀= −1.882214E−03 Tenth Surface k = −5.273886E−01, A₄ = −9.506179E−02, A₆= 6.538888E−03, A₈ = −7.519095E−04, A₁₀ = 1.760657E−05 Values of eachconditional expression are as follows: f12/f = 0.945 |f1/f2| = 0.460 νd1− νd2 = 19.5 f4/f = 0.641 Σd/f = 0.998 |f5/f| = 0.633

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theconditional expressions (1) to (6).

FIG. 14 shows the lateral aberration that corresponds to the half angleof view ω in the imaging lens of Numerical Data Example 5, and FIG. 15shows the spherical aberration SA (mm), the astigmatism AS (mm), and thedistortion DIST (%), respectively. As shown in FIGS. 14 and 15, in theimaging lens of Numerical Data Example 5, the image surface issatisfactorily corrected, and the respective aberrations aresatisfactorily corrected similarly to Numerical Data Example 1 to 4.

Accordingly, when the imaging lens of the respective embodiments isapplied to an imaging optical system of a cellular phone, a digitalstill camera, a portable information terminal, a security camera, anonboard camera, a network camera, and the like, it is possible to obtainthe high performance and the small size for the camera or the like.

Here, it is noted that the imaging lens of the invention shall not belimited to the above-described embodiments. For example, in the aboveembodiments, the fifth lens L5 is configured to have an inflection pointso as to restrain the incident angle of a light beam into an imagingelement. However, if there is some allowance in the incident angle of alight beam into the imaging element and it is not necessary to providean inflection point to the fifth lens L5, a lens surface of the fifthlens L5 may be formed in a aspheric shape that does not have aninflection point, or one surface or both surfaces of the fifth lens L5may be formed with a spherical surface(s).

The invention may be applicable to the imaging lens of a device that isrequired to have a small size and satisfactory aberration correctionability, e.g., the imaging lenses used in the cellular phones, thedigital still cameras, and the like.

What is claimed is:
 1. An imaging lens, comprising: a first lens havingpositive refractive power; a second lens having negative refractivepower; a third lens having negative refractive power; a fourth lenshaving positive refractive power; and a fifth lens having negativerefractive power in this order from an object side to an image side,wherein said first lens is a biconvex lens, said second lens has aconcave surface facing the object side, said third lens is a meniscuslens having a concave surface facing the object side, and said fifthlens is a biconcave lens.
 2. The imaging lens according to claim 1,wherein said first lens and said second lens have a composite focallength f12 and a whole lens system has a focal length f so that thefollowing conditional expression is satisfied:0.7<f12/f<1.4.
 3. The imaging lens according to claim 1, wherein saidfirst lens has a focal length f1 and said second lens has a focal lengthf2 so that the following conditional expression is satisfied:0.2<f1/f21<0.6.
 4. The imaging lens according to claim 1, wherein saidfirst lens has an Abbe's number νd1 and said second lens has an Abbe'snumber νd2 so that the following conditional expression is satisfied:15<νd1−νd2.
 5. The imaging lens according to claim 1, wherein saidfourth lens has a focal length f4 and a whole lens system has a focallength f so that the following conditional expression is satisfied:0.4<f4/f<1.0.
 6. The imaging lens according to claim 1, wherein saidfirst lens and said fifth lens are arranged so that a surface of thefirst lens on the object side is away from a surface of the fifth lenson the image side by a distance Σd on an optical axis and a whole lenssystem has a focal length f so that the following conditional expressionis satisfied:Σd/f<1.2.
 7. The imaging lens according to claim 1, wherein said fifthlens has a focal length f5 and a whole lens system has a focal length fso that the following conditional expression is satisfied:|f5/f|<1.0.
 8. An imaging lens, comprising: a first lens having positiverefractive power; a second lens having negative refractive power; athird lens having negative refractive power; a fourth lens havingpositive refractive power; and a fifth lens having negative refractivepower in this order from an object side to an image side, wherein saidfirst lens is a biconvex lens, said second lens has a concave surfacefacing the object side, said third lens is a meniscus lens having aconcave surface facing the object side, and said fourth lens has a focallength f4 and a whole lens system has a focal length f so that thefollowing conditional expression is satisfied:0.4<f4/f<1.0.
 9. The imaging lens according to claim 8, wherein saidfirst lens and said second lens have a composite focal length f12 and awhole lens system has a focal length f so that the following conditionalexpression is satisfied:0.7<f12/f<1.4.
 10. The imaging lens according to claim 8, wherein saidfirst lens has a focal length f1 and said second lens has a focal lengthf2 so that the following conditional expression is satisfied:0.2<|f1/f2|<0.6.
 11. The imaging lens according to claim 8, wherein saidfirst lens has an Abbe's number νd1 and said second lens has an Abbe'snumber νd2 so that the following conditional expression is satisfied:15<νd1−νd2.
 12. The imaging lens according to claim 8, wherein saidfirst lens and said fifth lens are arranged so that a surface of thefirst lens on the object side is away from a surface of the fifth lenson the image side by a distance Σd on an optical axis and a whole lenssystem has a focal length f so that the following conditional expressionis satisfied:Σd/f<1.2.
 13. The imaging lens according to claim 8, wherein said fifthlens has a focal length f5 and a whole lens system has a focal length fso that the following conditional expression is satisfied:|f5/f1<1.0.
 14. An imaging lens, comprising: a first lens havingpositive refractive power; a second lens having negative refractivepower; a third lens having negative refractive power; a fourth lenshaving positive refractive power; and a fifth lens having negativerefractive power in this order from an object side to an image side,wherein said first lens is a biconvex lens, said second lens has aconcave surface facing the object side, said third lens is a meniscuslens having a concave surface facing the object side, and said fifthlens has a focal length f5 and a whole lens system has a focal length fso that the following conditional expression is satisfied:|f5/f1<1.0.
 15. The imaging lens according to claim 14, wherein saidfirst lens and said second lens have a composite focal length f12 and awhole lens system has a focal length f so that the following conditionalexpression is satisfied:0.7<f12/f<1.4.
 16. The imaging lens according to claim 14, wherein saidfirst lens has a focal length f1 and said second lens has a focal lengthf2 so that the following conditional expression is satisfied:0.2<|f1/f21<0.6.
 17. The imaging lens according to claim 14, whereinsaid first lens has an Abbe's number νd1 and said second lens has anAbbe's number νd2 so that the following conditional expression issatisfied:15<νd1−νd2.
 18. The imaging lens according to claim 14, wherein saidfourth lens has a focal length f4 and a whole lens system has a focallength f so that the following conditional expression is satisfied:0.4<f4/f<1.0.
 19. The imaging lens according to claim 14, wherein saidfirst lens and said fifth lens are arranged so that a surface of thefirst lens on the object side is away from a surface of the fifth lenson the image side by a distance Σd on an optical axis and a whole lenssystem has a focal length f so that the following conditional expressionis satisfied:Σd/f<1.2.
 20. An imaging lens, comprising: a first lens having positiverefractive power; a second lens; a third lens; a fourth lens havingpositive refractive power; and a fifth lens, arranged in this order froman object side to an image side, wherein said first lens is arrangedopposite to the second lens, said first lens has a convex surface facingthe object side, said fifth lens has a concave surface facing the imageside, said concave surface being formed in an aspheric shape having aninflection point, and said first lens has an Abbe's number νd1, saidsecond lens has an Abbe's number νd2, said first lens has a focal lengthf1, and said second lens has a focal length f2 so that the followingconditional expressions are satisfied:15<νd1−νd20.2<|f1/f2|<0.6.
 21. The imaging lens according to claim 20, furthercomprising an aperture stop disposed on the object side of the firstlens.
 22. The imaging lens according to claim 20, wherein said secondlens has a concave surface facing the object side, and said second lenshas negative refractive power.
 23. The imaging lens according to claim20, wherein said third lens is formed in a meniscus shape and has aconcave surface facing the object side, and said third lens has negativerefractive power.
 24. The imaging lens according to claim 20, whereinsaid fifth lens is formed in a biconcave lens, and said fifth lens hasnegative refractive power.
 25. The imaging lens according to claim 20,wherein said first lens and said second lens have a composite focallength f12 and a whole lens system has a focal length f so that thefollowing conditional expression is satisfied:0.7<f12/f<1.4.
 26. The imaging lens according to claim 20, wherein saidfourth lens has a focal length f4 and a whole lens system has a focallength f so that the following conditional expression is satisfied:0.4<f4/f<1.0.
 27. The imaging lens according to claim 20, wherein saidfirst lens and said fifth lens are arranged so that the surface of thefirst lens on the object side is away from the surface of the fifth lenson the image side by a distance Σd on an optical axis and a whole lenssystem has a focal length f so that the following conditional expressionis satisfied:Σd/f<1.2.
 28. The imaging lens according to claim 20, wherein said fifthlens has a focal length f5 and a whole lens system has a focal length fso that the following conditional expression is satisfied:|f5/f|<1.0.
 29. An imaging lens, comprising: a first lens havingpositive refractive power; a second lens; a third lens; a fourth lenshaving positive refractive power; and a fifth lens, arranged in thisorder from an object side to an image side, wherein said first lens isarranged opposite to the second lens with a space in between, said firstlens has a convex surface facing the object side, said fifth lens has aconcave surface facing the image side, said concave surface being formedin an aspheric shape having an inflection point, and said first lens andsaid second lens have a composite focal length f12 and a whole lenssystem has a focal length f so that the following conditional expressionis satisfied:0.7<f12/f<1.4.
 30. The imaging lens according to claim 29, furthercomprising an aperture stop disposed on the object side of the firstlens.
 31. The imaging lens according to claim 29, wherein said secondlens has a concave surface facing the object side, and said second lenshas negative refractive power.
 32. The imaging lens according to claim29, wherein said third lens is formed in a meniscus shape and has aconcave surface facing the object side, and said third lens has negativerefractive power.
 33. The imaging lens according to claim 29, whereinsaid fifth lens is formed in a biconcave lens, and said fifth lens hasnegative refractive power.
 34. The imaging lens according to claim 29,wherein said first lens has a focal length f1 and said second lens has afocal length f2 so that the following conditional expression issatisfied:0.2<|f1/f2|<0.6.
 35. The imaging lens according to claim 29, whereinsaid fourth lens has a focal length f4 so that the following conditionalexpression is satisfied:0.4<f4/f<1.0.
 36. The imaging lens according to claim 29, wherein saidfirst lens and said fifth lens are arranged so that the surface of thefirst lens on the object side is away from the surface of the fifth lenson the image side by a distance Σd on an optical axis so that thefollowing conditional expression is satisfied:Σd/f<1.2.
 37. The imaging lens according to claim 29, wherein said fifthlens has a focal length f5 so that the following conditional expressionis satisfied:|f5/f|<1.0.